Polymer modified asphalts (PMAs) are useful for many applications, for example, but not limited to paving materials, roofing materials, products for protection of car bottoms and other coating applications. PMAs typically have good phase compatibility between the asphalt and the polymer, and should be stable at high temperature (storage stable) for ease of handling and application. For paving uses, it is desirable to have a PMA with a low viscosity at 135° C. (“135° C. viscosity”), for easy pumping, handling, and mixing at lower temperatures. These features translate into savings in energy costs and also minimize the danger of the polymer in the PMA decomposing at high temperatures. In specification AASHTO M320 (Superpave specification; [1]), it is indicated that the viscosity at 135° C. of paving asphalt should be no higher than 3000 mPa·s to ensure the pumping and handling of the PMA.
Several polymers have been identified that are effective modifiers for use in PMA preparations, in terms of weight percentage of polymer per increase of the Superpave high-temperature performance grade (AASHTO M320) [1], or in terms of improved rutting resistance. However, these known polymers often cannot be used, because they cause gelling, or are characterized as exhibiting a high 135° C. viscosity (i.e. above 3000 mPa·s).
In the manufacturing of PMAs, it is highly desirable to have the polymer molecules cross link to each other, forming a polymer network within the asphalt to provide hot-storage stability, improved rutting resistance and fatigue cracking resistance. A cross linking agent, such as sulfur, is generally added to the asphalt with the polymer to link up the polymer chains to form a network. However, the cross linking of the polymer molecules should not be so excessive that they no longer function as a liquid at high temperatures, but instead form semi-solids or gels. As sulfur, or an alternative cross linking agent, comes into contact with the polymer, some localized high concentrations of sulfur may cause excessive link-up of the polymer molecules and the formation of big lumps, which increases the 135° C. viscosity, and may even gel the resultant PMA.
A known method of producing PMAs involves high-shearing a polymer into an asphalt, and then adding a cross linking agent, such as sulfur or a sulfur containing compound, to create a polymer network within the asphalt (see for example U.S. Pat. No. 4,145,322, U.S. Pat. No. 4,330,449, U.S. Pat. No. 5,508,112, U.S. Pat. No. 5,719,216, U.S. Pat. No. 5,773,496 and Canadian patent 1,321,276). Following curing, the PMAs are typically stored at high temperatures (about 150° C.) before use. The methods disclosed in these patents work well with some asphalts, various types of polymers, and a number of different cross linking agents. However, several PMAs prepared using this method will gel after production or curing. Other PMAs prepared using this method are not storage stable and will gel after hot-storage of less than a week.
Depending on the type of polymer used, a PMA prepared using known methods may sometimes have high 135° C. viscosity, and also contain lumps, because of non-even cross linking of the polymer. The non-even cross linking is generally a result of contact of localized concentrations of sulfur or an alternative cross linking agent, with the polymer in the asphalt. In an attempt to solve this problem, many different approaches have been devised over the years including:
1. Gradual addition of sulfur, or an alternative cross linking agent, over a period of time, such as 12 hours, for example as disclosed in U.S. Pat. No. 6,429,241;
2. Use of multi-component or expensive cross linking agents, for example as disclosed in U.S. Pat. No. 5,508,112, U.S. Pat. No. 6,407,152, U.S. Pat. No. 6,451,886, U.S. Pat. No. 6,713,540, U.S. Pat. No. 6,956,071, and U.S. Pat. No. 7,081,492;
3. Use of a carrier oil to dilute the cross linking agent and polymer, for example as disclosed in U.S. Pat. No. 6,441,065;
4. Use of a mineral oil mixed with an activator to be added to the asphalt with the polymer to prevent a gel or lumps forming in the asphalt, for example as disclosed in U.S. Pat. No. 6,818,687; and
5. Use of a hydrocarbon dispersant, such as 2-ethylhexyl acid phosphate and oil, to be attracted to a polar cross linking agent, such as sulfur, for example as disclosed in U.S. Pat. No. 6,927,245.
Even though these methods may work for some PMA systems, the methods generally either involve a long and tedious sulfur addition process or the use of costly cross linking agents, and the need for extra carrier material, such as a dispersant chemical and oil.
U.S. Pat. No. 5,672,642 discloses a process for preparing a storage stable asphalt-polymer blend. The first step in the process involves contacting sulfur with asphalt to form a blend. A polymer, such as urethane, polyester, styrenic or olefin thermoplastic elastomer, is then added to the asphalt-sulfur blend to form a PMA.